Russia's Foreign Ministry has responded angrily to U.S.-led airstrikes on Syrian Army forces, calling them "on the boundary between criminal negligence and direct connivance" with Islamic State militants.

The incident, which Russia said killed 62 Syrian soldiers, sparked an angry exchange between diplomats at the United Nations.

It also threatened to further unravel a fragile cease-fire that went into effect on September 12 aimed at allowing humanitarian assistance into beseiged areas.

The Pentagon said in a statement that jets may have unintentionally struck Syrian troops while attacking Islamic State militants, but gave no further details. The Russian Defense Ministry said on September 17 that 62 Syrian government soldiers had died.

The incident was a result of Washington's "stubborn refusal" to cooperate with Moscow in fighting the Jabhat Fatah al Sham faction, and "other terrorist groups," the Russian Foreign Ministry said on September 18.

"The actions of coalition pilots -- if they, as we hope, were not taken on an order from Washington -- are on the boundary between criminal negligence and connivance with Islamic State terrorists," the ministry said.

A spokeswoman for the Russian Foreign Ministry suggested that the attack showed Washington was defending Islamic State, a charge that sparked an angry denunciation from the U.S. ambassador at the United Nations.

Later, Russia's U.N. representative Vitaly Churkin also downplayed that accusation, saying Russia had no "specific evidence" of the U.S. colluding with Islamic State militants.